A Hybrid Electrochemical/Chemical Synthesis of Zinc Oxide Nanoparticles and Optically Intrinsic Thin Films

A hybrid electrochemical/chemical (E/C) synthesis of zinc oxide (ZnO) nanoparticles and films is described. The E/C procedure involves two steps: Zinc metal was first electrochemically deposited at basal-plane oriented graphite electrode surfaces from dilute aqueous solutions; then this deposit was permitted to spontaneously oxidize and dehydrate at open circuit in the pH = 1.0 plating solution. Deposition was size-selective, and wurtzite phase ZnO nanocrystallites having mean diameters in the range from 15 to 100 Å were obtained using this approach. Relative standard deviations of the particle diameter for ZnO particle dispersions varied from 25 to 50%. Polycrystalline ZnO films of 100−400 Å in thickness were also obtained by depositing larger quantities of zinc metal in the first step of the synthesis. For ZnO particles (dia. < 80 Å), electron diffraction analysis revealed a preferred orientation for ZnO crystallites where the c-axis of the wurtzite unit cell was oriented perpendicular to the plane of the graphite surface, but X-ray powder diffraction data indicated that this orientational preference was lost when larger quantities of zinc were deposited and ZnO films were obtained. Luminescence spectra for the ZnO films prepared using this E/C method consisted of a single exciton band near 3.2 eV at room temperature with no deep trap state emission. At low temperatures (20 K), this exciton band split into a cleanly resolved and fully assignable phonon loss progression.